The Nikon N55 has a passive autofocus system.  These systems determine the distance to the subject by computer analysis of the image itself.  The camera looks at the scene and moves the back and forth searching for the best focus.  The point of the best focus is the point where there is maximum intensity difference and the maximum sharpness between adjacent pixels.  Therefore, the autofocus will not allow you to take a picture of a blank wall or a large object that is uniform in color because the camera can not compare adjacent pixels to find the best focus. 
The person taking the picture should still ultimately determine if the subject is in focus.  The autofocus is just there to assist you.  You must make sure the camera is not mistakenly focusing on another object or focusing on the background.

1.  If there is not enough light, the autofocus can not work properly and will not be able to focus.  If you are in a situation such as this, you should simply be able to press the shutter release button down lightly (not all the way) to automatically activate the Autofocus-Assist Illuminator.  This will allow the autofocus to focus on the subject you wish.